Re: "This is now merged to master with 64eaa2a, though it's fine to repeat this here"

The change is in here because I started out with the change from 64eaa2a and worked from there to this, and it was left as an artifact. With this commit, `content_length_` now only holds the value of the Content-Length response header (if it exists), and serves no other purpose anymore.  The initial value is actually never used.